1. Field of the Invention
The present invention relates to optical metrology, and more particularly to determining the azimuth angle accurately for an oblique Spectroscopic Ellipsometry (SE) or an unpolarized or polarized spectroscopic reflectometer device.
2. Description of the Related Art
In the manufacture of integrated circuits, very thin lines or holes down to 10 nm or sometimes smaller are patterned into photoresist and then often transferred using an etching process into a layer of material below on a silicon wafer. It is extremely important to inspect and control the width and profile (also known as critical dimensions or CDs) of these lines or holes. Traditionally the inspection of CDs that are smaller than the wavelength of visible light has been done using large and expensive scanning electron microscopes. In many cases, however, manufacturers would like to have measurements immediately after the photoresist has been patterned or etched to have tight control of the process before it drifts out of spec. Testing the wafer early during production and controlling the fabrication steps according to the test results helps to keep production costs low and to keep yields high. Ideally, the measurement tool would be integrated into the wafer track that develops the photoresist or integrated into the wafer-etching tool.
In typical stand-alone instruments, the wafer is moved on a stage, while the measurement optics remains stationary. In addition, when the angle of incidence on the wafer is other than zero (e.g. in an ellipsometer), the wafer is preferably oriented so that the plane of incidence is perpendicular to the lines of gratings on the wafer.
One general technique that has promise for integrated CD measurements is Scatterometry. This technique takes advantage of the fact that an array of small lines or holes affects the properties of the light in the zero order that is reflected (and, for transparent samples, transmitted) from such an array. Various measurable properties of the zero-order light will vary depending on the dimensions of the structure on the wafer. Often such parameters are measured versus wavelength, and in some cases, versus angle of incidence on the sample. Normal-incidence spectroscopic reflectometers show particular promise because they can be used with the wafers in any arbitrary orientation. Typically, CD measurements have been made using instruments such as an ellipsometer or reflectometer that were originally designed to measure film thickness. The data from such instruments is usually fed to a processor, which analyzes the measurements, usually by accessing a library of theoretically generated data for a range of array dimensions and film properties near those of the expected dimensions of the sample. The measured data are compared to the library and a best-fit match to a data set in the library is found. The processor then outputs the corresponding dimensions.
Since there are multiple unknown variables that may need to be measured, such as line width, line edge slope, top film thickness, underlying film thickness, or film refractive index, it is desirable that the measurement technique measure as many multiple independent parameters as is practical. It has been shown that there are only 4 independent measurable quantities for a given wavelength and incident condition (angle of incident and azimuth angle). Namely, these 4 measurables are intensity reduction (one independent parameter), and polarization state change (3 independent parameters). Thus, the measurement instrument is improved to measure over a wider wavelength range, and attempts to cover all the four measurable quantities. These techniques are described in U.S. Pat. No. 7,064,829, entitled “Generic Interface for an Optical Metrology System”, by Li, et al., issued on Jun. 20, 2006, which is incorporated in its entirety herein by reference. Coulombe et al. (‘Ellipsometric-Scatterometry for sub-0.1 mm CD measurements,’SPIE, Vol. 3332, p. 282-292) investigated reflectometry and ellipsometry of line gratings as a function of angle of incidence and azimuth.
Optical metrology involves directing an incident beam at a feature on a wafer, measuring the resulting diffraction signal, and analyzing the measured diffraction signal to determine various characteristics of the feature. In semiconductor manufacturing, optical metrology is typically used for quality assurance. For example, after fabricating a periodic grating in proximity to a semiconductor chip on a semiconductor wafer, an optical metrology system is used to determine the profile of the periodic grating. By determining the profile of the periodic grating, the quality of the fabrication process utilized to form the periodic grating, and by extension the semiconductor chip proximate the periodic grating, can be evaluated.
An integrated CD measurement tool must be both fast and compact, and must not damage the wafer under test. The wafer might rotate, to have a preferred measurement orientation with respect to certain wafer features and to compensate the wafer load tolerance. The wafer may also be loaded into the measurement tool at an arbitrary angle creating further complications for instruments that have a preferred measurement orientation with respect to certain wafer features.
There is a need to increase the tool available time and decrease the maintenance time associated with integrated metrology tools. There is also a need to design integrated metrology tools for measuring CDs and overlay error on periodic structures that are compact and well suited for integration into a wafer process tool.